The extracellular matrix consists of a complex network of varied molecules of protein or carbohydrate origin, that are responsible for an environment favorable to correct cellular functioning.
During cellular events such as proliferation, migration or differentiation, the extracellular matrix modifies and adapts. In the polysaccharide family, glycosaminoglycans (GAGs), and among these hyaluronic acid (HA), are key structural components of this extracellular matrix. These GAGs, which have very high molecular weights, possess many physiological functions, such as moisturizing tissues, increasing the skin barrier effect, and promoting the resilience and elasticity of tissues by virtue of their ability to trap water. Moreover, they have been described in the modulation of angiogenesis or neocreation of blood capillaries, of healing, of proliferation, of migration and of cell differentiation by means of membrane receptors such as CD44 and RHAMMs (receptor for hyaluronan mediated motility) located at the cell surface (Lesley J. et al., J. Exp. Med., 175, 257-266 (1992); Peck D. et al., Journal of Cell Science, 111, 1595-1601 (1998); Assmann V. et al., Journal of Cell Science, 111, 1685-1694 (1998)).
Moreover, the presence of hyaluronic acid at the dermal-epidermal interface is thought to make it possible to facilitate metabolic exchanges between the two tissues. Finally, hyaluronic acid also allows associations with other molecules such as versican, fibrinogen and collagens type I and VI (Mc Devitt et al., FEBS Lett., 3, 294, 167-170 (1991); Le Baron et al., J. Biol. Chem., 14, 267, 10003-10010 (1992)).
Hyaluronic acid, which has been more thoroughly studied than the other GAGs, is like a linear polysaccharide composed of repeating disaccharide units: N-acetyl-D-glucosamine-β(1→4)-D-glucuronic acid β(1→3). It is synthesized on the inner side of the plasma membrane by successive additions of UDP-N-acetylglucosamine and of UDP-D-glucuronic acid combined via β(1→4) and β(1→3) linkages, and then secreted directly into the extracellular space (Weigel et al., J. Biol. Chem., 272, 13997-14000 (1997)). This synthesis is carried out by monomeric membrane-bound enzymes called hyaluronan synthases (HASs), identified as three isoforms in vertebrates: HAS1, HAS2 and HAS3 (Weigel et al., J. Biol. Chem., 272, 13997-14000 (1997)) transcribed from 3 distinct genes located on various chromosomes. More particularly, the gene encoding the HAS2 protein is located on chromosome 8 and is sensitive to the presence of various growth factors such as PDGF-BB, TGF-β1, EGF or else FGF and KGF.
GAGs are found in approximately all tissues and their amounts in the extracellular matrix are greatly modified under certain conditions such as skin atrophy, age, atrophy scars, and probably osteoporosis. The skin, and more particularly the dermis, contains approximately 50% of the total amount of hyaluronic acid present in an organism (Laurent et al., FASEB J., 6, 2397-2404 (1992)) and studies have shown a strong cause-effect relationship with respect to age, with a complete absence of hyaluronic acid synthesis after 60 years of age (Ghersetich et al., Int. J. Dermatol., 33, 119-122 (1994)). This hyaluronic acid is synthesized by the HASs of dermal fibroblasts.
At the epidermal level, hyaluronic acid is synthesized by keratinocyte HASs and plays a role in the migration, proliferation and differentiation thereof. The decrease in hyaluronic acid leads to a thinning of the epidermis which can be attributed to the acceleration of keratinocyte terminal differentiation.
For cosmetic or dermopharmaceutical applications, a certain number of compounds have been studied for their ability to stimulate, in keratinocytes, the production of glycosaminoglycans in general, and of hyaluronic acid in particular.
After having demonstrated that HAS2 can be induced by various substances such as TNF-alpha, interferon-gamma, interleukin-1 beta (Ijuin et al. 2001, Arch. Oral Biol. 46(8): 767-72), various teams have attempted to stimulate HAS2 production with substances such as retinoic acid, on a model of keratinocytes in culture, in order to evaluate the effects on the quality and the structure of the epidermis of skin from normal volunteers thus treated. The conclusion of these studies is that all the pro-inflammatory cytokines stimulate the production of HAS2 and therefore of hyaluronan in cells in culture (Jacobson A, Brinck J, Briskin M J, Spicer A P and Heldin P. 2000. Expression of human hyaluronan synthases in response to external stimuli. Biochem. J. 348(Pt 1): 29-35). Thus, substances such as retinoic acid, recently, ginseng substance K (or 20 O-beta-D glucopyranosyl 20 S protopanaxadiol) has recently been described as being capable of stimulating HAS2 synthesis in transformed keratinocyte-type HaCat cells or in human fibroblasts in culture (Kim et al., IFSCC Magazine 7(3), 189-196 (2004)).